Most current commercial methods for producing anhydrous alumina powders involve comminution steps whereby the initial powder particles or agglomerates are broken down to a desired size range.
The most commonly desired form of alumina for ceramic article fabrication is a very fine powder. The crystals of alpha alumina are extremely hard and durable and hence quite difficult to mill to fine sizes; the process requires long milling times and is energy intensive. It is possible to mill hydrated aluminas (such as are produced in the Bayer process) more easily than the alpha form, however, sodium contamination is generally a problem and agglomeration arises when the hydrate is calcined to alpha and further milling is required. A major reason that this agglomeration occurs is that &gt;1100.degree. C. is required to convert the hydrated form to alpha; at this temperature partial sintering and grain growth occur.
A key feature in the millability of alpha aluminas is the size of the ultimate crystals since sintered bonds are weaker than atomic crystalline bonds. It follows then that in milling alpha powders, it is desirable to comminute the agglomerated systems rather than ultimate crystals. Clearly then, the ultimate crystal size should be equal to or less than the desired milled particle size to minimize time and energy requirements for the size reduction process.
One of the major problems with current comminution technology of alpha aluminas is this ultimate crystal size factor. Bayer process derived alpha alumina is produced from the hydrated form and is therefore necessarily calcined at 1100.degree. C. or more. It is difficult to obtain submicron ultimate crystals in this process, particularly if sodium removal at high temperatures is also carried out. In order to manufacture a submicron alpha powder, the ultimate crystals themselves must be ground to a finer size, a difficult task.
According to the present invention we have developed techniques for producing extremely fine (&lt;0.5 micron) ultimate crystals of alpha alumina; such aluminas have the desireable properties of being relatively easy to reduce in size to their ultimate crystal size and, being highly reactive, sinter at lower than conventional temperatures by virtue of their small particle size (high surface energy).
It is, accordingly, an object of the present invention to produce alumina (preferably the alpha form) with ultimate crystal size less than one micron (preferably less than 0.5 micron) which can be deagglomerated, as by milling, at much less than conventional milling times and energies and will be much more thermally reactive (i.e. easily sinterable) than conventional powders, and to produce fired monolithic bodies therefrom.